This invention relates to the production of electrodes for use in molten carbonate fuel cells. In particular it is concerned with anode structures that tend to creep or otherwise distort within the loaded conditions of a fuel cell stack. A typical fuel cell stack for commercial or utility use may contain hundreds of electrodes.
Fuel cells with alkali-metal carbonates as electrolyte are well known and are generally referred to as molten carbonate fuel cells. Such fuel cells and stacks of cells are illustrated and described in U.S. Pat. No. 4,514,475 to Mientek; U.S. Pat. No. 4,411,968 to Reiser et al. and U.S. Pat. No. 4,206,270 to Kunz et al.
Molten carbonate fuel cells have used porous plaques of nickel and nickel alloy as anode structures. These anodes tend to be dimensionally unstable losing thickness by creep distortion within the fuel cell stack. It is well accepted to use chromium additive into the nickel anodes to enhance the structural stability of the plaque. In U.S. Pat. No. 4,239,557 to Thellmann et al., nickel anodes with up to 30 weight percent chromium are disclosed as being thermally stable at elevated temperatures. Typically alloys of at least 10 weight percent chromium are used to stabilize a porous nickel structure for use as a molten carbonate fuel cell anode.
The use of such high levels of chromium in porous nickel anodes not only is expensive but may result in the degeneration of the structure on oxidation of the chromium. Prior efforts to reduce the amount of chromium to less than 5% by weight have resulted in nickel anodes with increased susceptability to creep under fuel cell stack conditions.
Various efforts have been made to stabilize anode structures with low chromium concentrations. Precipitation hardening with elements such as aluminum or titanium in small proportions, solid solution strengthening by impregnating a standard nickel anode with a solution of the strengthening element such as aluminum, copper, tin or chromium and strengthening by second phase dispersed particles, such as CeO.sub.2 or Cr.sub.2 O.sub.3 have been investigated. In some instances promising results have occured. However, in long term operations under simulated fuel cell stack conditions of a hundred hours or more, the initial creep resistance and stability of the anodes have degraded.
Therefore, in view of the above discussion it is an object of the present invention to provide a method of forming a dimensionally stable electrode structure for molten carbonate fuel cell use.
It is further object to provide a method of preparing a porous anode of nickel and chromium that maintains structural stability in extended use under fuel cell stack conditions.
It is also an object to provide an improved method of producing a porous nickel-chromium alloy with a substantially reduced chromium content over that previously required for long-term stability.
In accordance with the present invention, a method is disclosed for forming a dimensionally stable electrode structure for use in a fuel cell with molten alkali metal carbonate as an eletrolyte. A porous plaque of a nickel-chromium alloy with no more than 5 weight percent chromium is prepared. The chromium is selectively oxidized by exposure to a steam-hydrogen gas mixture containing only a minor proportion of hydrogen in respect to a major proportion of steam at a temperature of at least 600.degree. C. but not more than 800.degree. C.
In further aspects of the invention, the cromium in the plaque is selectively oxidized by exposure to a steam-hydrogen gas mixture at a temperature of 700.degree.-800.degree. C. for at least one hour.
In yet other aspects of the invention, the steam-hydrogen gas is provided in mixture with an inert carrier gas.
In another important aspect of the invention, the steam to hydrogen gas mixture is provided in a volumetric ratio of 80/1 to 120/1 preferably in a steam to hydrogen ratio of about 100/1.
In other aspects of the invention, the porous plaque of nickel has a porosity of about 50 to 60%, a composition of about 98% nickel and 2% chromium by weight, and is formed as a nickel-chromium alloy by heat treating a particulate mixture of nickel and chromium metals of similar particle size for about 5-90 minutes at a temperature of about 1000.degree.-1100.degree. C. preferrably about 1050.degree. C. for about 15 minutes.